1. Field of the Invention
The present invention relates in general to V-type 8-cylinder four cycle internal combustion engines with a bank angle of 90 deg., and more particularly to the engines of a type that has a double link type piston-crank mechanism that employs a plurality of links for operatively connecting a crankshaft and each piston.
2. Description of the Related Art
Hitherto, as a means for providing the engine with a variable compression ratio, there has been proposed a type that practically uses a double link type piston-crank mechanism. The mechanism comprises an upper link that has one end pivotally connected to a piston through a piston pin, a lower link that is pivotally connected to the other end of the upper link and pivotally supported by a crankpin of a crankshaft, and a control link that has one end pivotally connected to the lower link for controlling the posture of the lower link. In accordance with an operation condition of the engine, the other end of the control link, that forms a swing fulcrum, is forced to change its position. With this, the posture of the lower link is varied and thus, a stroke characteristic of the piston is changed permitting the engine to have a variable compression ratio.
For controlling such engines, one operation method has been hitherto proposed wherein when the engine is under a low operation load, a higher compression ratio is set for improving the fuel consumption and when the engine is under a high operation load, a lower compression ratio is set for suppressing an excessive pressure generated in each cylinder. By practically using this method, a unique system has been thought out wherein the compression ratio is controlled to vary in accordance with the engine operation condition. In internal combustion engines employing such system, both reduction in fuel consumption and increase in engine power are achieved at the same time.
In the engines having the above-mentioned double link type piston-crank mechanism installed therein, it is known that a secondary vibration component of an inertia force produced by reciprocating movement of each piston is reduced, as is described in Japanese Laid Open Patent Application (Tokkai) 2001-227367. This advantageous effect is brought by a multi-articulation possessed by the double link type mechanism through which the piston and the crank pin are operatively connected. It has been revealed that a mechanism for moving the position of the swing fulcrum of the control link has substantially no influence on such advantageous vibration reduction effect.
For effective reduction of the secondary vibration component of the inertia force of the piston, various methods have been proposed and put into practical use, which are disclosed in, for example, the above-mentioned published Application 2001-227367, Japanese Laid-open Patent Application (Tokkai) 2002-227674 and Japanese Laid-open Patent Application (Tokkai) 2002-129995.